This invention relates to refrigeration systems, and in particular to a method and apparatus for ensuring maximum utilization of a refrigeration system. While the invention is described in detail with respect to a conventional refrigeration or air conditioning system, those skilled in the art will recognize the wider applicability of the invention disclosed hereinafter. The invention may find application, for example, with heat pumps, air conditioning, refrigeration systems, or other devices where system efficiency may be improved by monitoring specific parameters affecting that efficiency.
The operational features of a conventional air conditioning system are well known in the art. In general, such systems include a compressor which forces the particular refrigerant used in the system through a condensing coil, an expansion valve, an evaporation coil, and back to the suction side of the compressor. The expansion valve plays an important part in the overall efficiency exhibited by the system. Under ideal operating conditions, the expansion valve should admit an amount of refrigerant that can be evaporated and slightly super-heated in the evaporator coil. That is to say, the evaporator coil should be "wetted" along approximately its entire length to provide a good heat transfer rate and maximum system efficiency. In the past, some portion of the evaporator coil always has been dry. A dry evaporator coil portion was utilized in order to prevent the passage of liquid to the suction side of the compressor. Liquid entering the suction side of the compressor causes damage to the compressor valves. Consequently, it is the prevalent practice to design a system with a safety margin so that at light load conditions, the coil is operating at its most efficient point, in that maximum coil length is available for heat transfer. However, as the load increases, the length of coil available for effective heat transfer decreases so that heavy load conditions represent the least efficient operating area.
Thermostatic control valves presently are the most prevalent means for controlling the operation of a refrigeration system. Thermostatic control valves generally include a diaphragm actuated valve member having one side of the diaphragm operatively connected to a pressure generating means. The pressure generating means commonly is a sealed bulb or sensor having a gas responsive to temperature enclosed in it. The opposite side of the diaphragm is opposed by system pressure and the diaphragm is preloaded by means of a spring to set the operating point of the valve. Volume changes in the gas of the sensor, in response to changes in temperature, operate the valve. While these devices work well for their intended purposes, the thermostatic expansion valve cannot adequately improve system efficiency by assuring full utilization of the refrigeration coil, because a comparatively small system gain must be used to regulate system stability.
Thermostatic control valves suffer an additional disadvantage in heat pump applications. As will be appreciated, a heat pump, for explanational purposes, may be considered a reverse cycle refrigeration system. Consequently, two thermostatic expansion valves must be used, since a different adjustment of the valve normally is required for each coil. Thermostatic expansion valves also generally control flow only in one direction through the valve. Such duplication in heat pump applications results in increased cost.
As indicated above, under ideal operating conditions, the expansion valve should emit just the amount of refrigerant that can be evaporated and slightly super heated. That is to say, the evaporator shoud be wetted to the maximum extent so that optimum heat transfer results. Generally, it is known that the temperature along an evaporator coil, for example, decreases from the inlet to outlet more or less proportionally with the distance of a point along the coil from the inlet. At the point on the coil where super heating begins, the temperature reverses and begins to rise rapidly to the outlet of the coil. In a flooded coil, the temperature continues to drop throughout the coil while in a starved coil, the notch or temperature inversion moves toward the start of the coil.
The invention disclosed hereinafter permits the precise control of the expansion valve so that the coil can be operated safely in its flooded state, without the fear of damage to the compressor structure. In the alternative, the area of super heat can be controlled to such an extent that essentially the entire coil is available for heat transfer. This degree of control is achieved by measuring the temperature between two points on the coil, the two points being selected arbitrarily to give optimum efficiency, and by controlling fluid flow through the expansion valve to make the temperature at the first point equal to the temperature at the second point. As an optional feature, it is possible to bias the temperature sensed at the two points through the control circuit so that the temperature differential between the points becomes a function of a set of independent variables.
The control feature is stated mathematically as F(X) = .DELTA.(T.sub.1 -T.sub.2). When the coil is starved, T.sub.2 is larger than T.sub.1, and the difference T.sub.1 -T.sub.2 is a negative signal which is used to drive the expansion valve to a more open condition, until the temperature differential between T.sub.1 and T.sub.2 is again 0. When the coil is flooded, T.sub.1 is greater than T.sub.2, so that the temperature differential T.sub.1 -T.sub.2 produces a positive signal, which again may be used to drive the expansion valve toward its closed position, to reduce the fluid flow until the temperature differential again approximates to 0. If the temperature differential T.sub.1 -T.sub.2 is maintained at 0, super heat in the coil always will be ideal and independent of any load on the evaporator coil.
One of the objects of this invention is to provide an improved means for controlling a refrigeration cycle of an apparatus.
Another object of this invention is to provide a method of operating a refrigeration system in which the evaporator coil is capable of operation at a completely saturated or wetted condition.
Another object of this invention is to provide a refrigeration system which employs electrical modulation of the evaporator and condensing coil.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.